The present invention relates to a cylindrical standard shank and method for rotating cutting tools and, more particularly, to a cylindrical standard shank used in industrial large-scale machining production with transfer lines.
In industrial large-scale machines applications of this type, the many work spindles to be tooled are preferably provided with standardized location openings for direct location of the cutting tools in order to avoid expensive chucks, a loss of clamping accuracy and avoid true-running errors. A high true-running accuracy is important in order to be able to achieve long tool life of the cutting tools. However, so that the cutting tools, which vary quite a lot in size, can be inserted into the standardized location openings, a standard shank serving as an adaptor must in each case be attached to the tool shank of each cutting tool. In such arrangement, the mutual connection must, of course, be realized with the greatest possible true-running accuracy and high clamping force and in a low overall volume.
In the case of conventional standard shanks, the cutting tool must be brazed or adhesively bonded into place in the location bore of the standard shank. Neither the brazed nor the adhesively bonded connection can be released again in such a way that the standard shank could be reused. Alternatively, the cutting tool must be pressed by a very tight press fit into the standard shank location bore, which is produced undersize, or the location bore of the standard shank must be shrunk onto the shank of the cutting tool by heating and subsequent cooling, so that the cutting tool can no longer be non-destructively removed therefrom.
The abovementioned locating techniques for a cutting tool are certainly very space-saving and have sufficient true running accuracy if the working surfaces of the cutting tool are ground in the tool grinding machine only after the cutting tool is inserted and fixed in the standard shank and upon location of the entire tool on the joining cylinder of the standard shank. In all these cases, however, the standard shank must also be scrapped after complete wear or fracture of the cutting tool. In addition, brazing the cutting tool in place can only be realized in the case of certain tool materials, namely carbides, and even in the case of these materials only by tolerating certain structural impairments.
Clamping the cutting tools in the standard shank with a collet, although conceivable, would not be suitable in practice, since such clamping can reproduce only too low a true-running accuracy, produces inadequate fixing of the cutting tool in the standard shank, and could be realized at adequate stability only with a large overall volume, especially radially.
The same reasons also apply in principle to the clamping of the cutting tools in a standard shank with a metal collar which can be acted upon hydraulically (cf., e.g., German Offenlegungsschrift 3,909,630 or German Patent No. 2,639,320), where the disadvantages of a large overall volume and a cutting tool mounting arrangement, floating within limits, especially becoming apparent. Furthermore, uniformly maintaining the hydraulic clamping pressure at a high level over the entire life of the cutting tool is here very problematic and is not truly achieved in practice.
It would also be conceivable to effect the clamping of the cutting tool in place in the standard shank with a so-called whistle-notch connection, as is also provided for clamping the standard shank in place in the work spindle of the machine tool. Such a whistle notch is formed by a sloping face on the shank periphery, which sloping face is slanted by about 2.degree. to the inside and on which a grub screw acts which is arranged orthogonally thereto in the wall of the work spindle. This joining technique is, however, only suitable for larger shank diameters. Besides, for spindle-side clamping, it represents, on one hand, an unavoidable compromise between the demands for high true-running accuracy, here only just justifiable, and quicker as well as simpler releasability of the clamping using, on the other hand, available conventional workshop techniques.
Using a whistle-notch connection once again in the same tool-clamping arrangement would allow the unavoidable true-running errors to accumulate to an inadmissibly large degree. In addition, the connection between cutting tool and standard shank is not to be releasable with simple workshop techniques. For these reasons, the use of so-called whistle-notch connections between the cutting tool and standard shank is not suitable.
An object of the present invention is to improve the standard shank and to provide a method to the effect that the cutting tool can certainly be easily located, in a releasable manner, to be clamped, and consequently the standard shank can easily be used again repeatedly. Nonetheless, with a small overall volume of the clamping device, the cutting tool can be held in position with high retaining force and at a high, reproducible true-running accuracy in accordance with the present invention. In addition, the tool life is on no account to be impaired by the nature of the novel tool clamping but is to be advantageously influenced if at all possible.
This object has been achieved according to the invention by providing that unitarily connecting the standard shank having a thin-walled clamping sleeve in the front axial area of the location bore to a remaining portion of the standard shank and provided at a periphery thereof with a cylindrical outer surface of defined outside diameter, pushing a shrink collar of a shape-memory alloy configured to be onto the clamping sleeve, an axial length of said shrink collar approximately corresponding to, at most, an axial length of the clamping sleeve wherein the shape-memory alloy is selected with regard to its composition (alloy type) such that an its austenite finishing temperature thereof is below a working temperature, and passing the shrink collar through manufacture-induced states to achieve an intended function thereof. These states include working an inner surface of the shrink collar facing the clamping sleeve starting from an austenitic structural state to a clear undersize relative to an outside diameter of the clamping sleeve so that high pre-tension theoretically results when the shrink collar is paired with the outer surface of the clamping sleeve, expanding the shrink collar, subsequently transformed by undercooling into a martensitic structural state, in the peripheral direction such that the diameter of the inner surface of the shrink collar in the martensitic structural state has a slight oversize relative to the clamping sleeve, and selectively displacing the shrink collar on the clamping sleeve.
Accordingly, the releasability of the cutting tool from the standard shank is provided through the use of the collet principle, which is known per se but for several reasons is inadequate in conventional design, for the present tool clamping. Nonetheless, the high clamping force which can thereby be achieved and the reproducibly high clamping accuracy at a small radial overall volume surprisingly result from the use of a shrink collar of known shape-memory alloy. On account of the high clamping pressure uniformly effective over the entire intrados (inner arch) surface of the clamping sleeve, the accuracy of the intrados surface can become fully effective during the tool clamping.
During transformation, produced by heating, of the material structure of the shrink collar from martensitic (undercooled) into austenitic (room temperature), the shrink collar shrinks in diameter on account of the shape memory of the alloy. In the course of this shrinkage, the collar bears on the outside of the clamping sleeve with high force. Due to the high true-running accuracy, residual out-of-balance effects and thus the excitation of out-of-balance vibrations of the tool are very slight, which has an advantageous effect on the machining accuracy and the tool life.
In addition, the tool clamping arrangement according to the present invention has proved to have an astonishingly good vibration-damping characteristic, which likewise improves the machining accuracy and the tool life. The vibration-damping effect comes about, on one hand, through the two concentric, cylindrical fitting-surface pairs on account of a clamping hysteresis and, on the other hand, on account of the good damping property of the shrunk collar shape-memory material itself.
When the invention is widely used in large-scale machining production of a large company, a great number of advantages are obtained. For instance the standard shank can easily be used again upon fracture or after complete utilization of the regrinding reserve of the cutting tool located therein. On account of the standard shank roughly twice as expensive by itself compared with the unit price of the cutting tool and on account of the reusability of the standard shank, there is a great savings potential when there are a large number of standard shanks in a large-scale production. Despite the releasability of the cutting tool, it can be clamped in place in the standard shank or held in position therein at a reproducibly high true-running accuracy and high retaining force. Since the tool clamping takes place at temperatures at which the material structure of all conventional cutting-tool materials remains completely unchanged (i.e. low temperatures) and also no phases of high thermal expansion are passed through, tools made of all conceivable materials, in particular the relatively inexpensive high-speed cutting steel or the especially wear-resistant cutting ceramics, can be clamped with the standard shank according to the present invention.
Compared with conventional change clamping tools, the standard shank of the present invention provides (1 ) an unequalled high retaining force; (2 ) an unequalled high true-running accuracy (&lt;15.mu.m) which can be reproduced even after repeated clamping; (3) an unequalled low overall volume, particularly in a radial respect, which is especially important for machining in restricted surroundings of a workpiece; (4) high damping action of the shrink collar, made of shape-memory alloy, against radial vibrations; (5) independently thereof, a damping effect through two joining surface pairs, lying concentrically one inside the other, on account of a clamping hysteresis; (6) on account of the high damping effect of the tool-clamping arrangement against radial vibrations of the tool, longer tool life can be achieved with this tool-clamping arrangement than with known tool-clamping arrangements having less effective damping; (7) the clamping connection between cutting tool and standard shank can be released relatively simply undercooling by placing in dry ice (CO.sub.2) or immersing in liquid nitrogen, but it cannot be released, not even inadvertently, by the machine operator of a transfer line using the conventional workshop techniques in-situ there, so that there is a tool-clamping arrangement which is reliable for a long period; (8) standardized cutting tools, which are thus less expensive and can subsequently be delivered at short notice, can be located and fixed in the already finish-ground state in the standard shank by the customer or user of the tools so that costly stock-keeping, planned for the long term, of expensive special tools at the user's can be dispensed with; and (9) the standard shank, if it is hollow-drilled, permits reliable and loss-free conduction of cooling and lubricating emulsion without a separate seal through the clamping arrangement of the cutting tool.
German Offenlegungsschrift 3,007,307, for example, certainly discloses reversibly releasable shrink connections with the use of shrink collars of shape-memory alloy, but here this concerns the connection of abutting cylindrical parts, in particular tubes, which are connected by a shrink collar resting directly on the two tube ends and bridging the joint. In view of the high accuracy requirements for tool-clamping arrangements, it could not be foreseen by the person skilled in the art and active in this field that the shrink connection known from the field of steel construction, when applied to a collet, which does not produce the necessary clamping accuracy for the present use, is nonetheless completely satisfactory here with regard to all requirements to be met. Therefore, application to a completely different field in a modified use of the shrink collars of shape-memory alloy must be considered to be original, technically unconventional and outstanding.
By the length restriction of the longitudinal slots of the clamping sleeve, the clamping arrangement of the tool shank of the present invention is to be effectively sealed off against loss or premature escape of cooling and lubricating emulsion.